1. Field of the Invention
The present invention relates to the amplification technology of electric signals, and more particularly to the technology of an amplifier circuit whose gain can be varied.
2. Description of the Related Art
FIG. 1 is explained. FIG. 1 is one example of a variable gain amplifier circuit whose gain can be changed.
In FIG. 1, the non-inversion input terminal of an operational amplifier OP1 is connected to the ground (reference potential, virtual earth) of the circuit. A feedback resistor R0 is connected between the output terminal and inversion terminal of the operational amplifier.
An input resistor R1 is connected between the input terminal IN of this variable gain amplifier circuit and the inversion input terminal of the operational amplifier OP1. The resistance value of this input resistor R1 can be changed.
An amplifier circuit with a configuration such as that shown in FIG. 1 is a typical inversion amplifier circuit. It is widely known that the gain G of the inversion amplifier circuit is as follows (here, no inversion of a signal is considered):G=V0/V1=R0/R1In this case, when changing the value of the input resistor R1, the gain of the circuit varies. FIG. 2 is a graph showing the change of the frequency and phase characteristics of the circuit shown in FIG. 1 when changing its gain thusly.
As seen from FIG. 2, when reducing the gain of the circuit shown in FIG. 1 by increasing the value of the input resistor R1, the phase characteristic also varies and frequency at which a phase slews shifts towards the higher frequency side. This is because the amount of feedback from the output of the operational amplifier OP1 to the inversion input terminal is changed by increasing the value of the resistor R1. Such a phase characteristic change can cause distortion in the waveform of a signal amplified by the circuit.
In the circuit shown in FIG. 1, the amount of feedback from the operational amplifier OP1 to the inversion input terminal is as follows:V0×R1/(R1+R0)
Concerning the present invention, for example, Japanese Patent Application No. H8-116224 discloses a technology for reducing gain error that is due to the conduction resistor for a gain changing-over switch and reducing the total resistance value of resistors used for creating gain change by making the ratio between the input resistor and the conduction resistor for a dummy switch that is connected to it in series and is in the normal conduction state almost the same as the ratio between a gain setting resistor, which is a feedback resistor, and the conduction resistor for a gain changing-over switch in the inversion or non-inversion amplifier that uses an operational amplifier.